1. Field of the Invention
The present invention relates to a process for the production of an expansion-molded article.
2. Description of the Related Art
It has been known to produce a molded article (so-called expansion-molded article) by filling prefoamed thermoplastic particles in a mold cavity formed by a combination of mold sections, through each of which plural heating medium holes are defined, and then heating the thus-filled particles with a heating medium such as steam to fusion-bond them into the article conforming with the mold cavity. In the mold employed upon production of the molded article, the heating medium holes which may hereinafter also be called "steam holes" are generally formed by fitting parts, which define plural slits or small bores permitting passage of steam therethrough and are called "core vents", in through-holes formed in the mold. Construction of such steam holes by fitting removable parts such as core vents in the through-holes of the mold is needed because the resin tends to stick in the steam holes and hence to cause clogging of the steam holes as molding is repeated. When the steam holes are clogged, the construction of the steam holes by removable parts such as core vents permits elimination of the clogged resin after removing the core vents from the mold. This assures better workability than the elimination of clogging of steam holes when the steam holes are formed directly in a mold.
Use of such core vents is however still accompanied by a serious problem in the efficiency of production as the core vents must be removed from the mold usually every 7,000-10,000 shots (at intervals of 20-30 days for industrial scale production) and then heated to 200.degree.-300.degree. C. to fuse off the resin clogging the steam holes of the core vents and the molding work has to be halted during this period. Further, many core vents each of which has plural steam bores are distributed on the surface of a mold. Numerous marks of these steam bores of the core vents are therefore remain on the surface of the resulting molded article, leading to another problem such that the commercial value of the molded article is reduced.